Linear optical scanner

ABSTRACT

A linear optical scanner wherein the linear optical scanner is disposed between a lens and the image plane of the lens allowing the image from the lens to be optically shifted along one dimension, and permits the image from the lens to be shifted linearly without scan error.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional PatentApplication Serial No. 60/232,475 filed Sept. 13, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to a linear optical scanner for usein scanning two dimensional and three dimensional objects, and moreparticularly to a linear optical scanner having a three-mirrorconfiguration causing the image from a lens to be shifted across alinear array of one-dimensional detectors of a linear array camera.

BACKGROUND OF THE INVENTION

[0003] Prior art methods of scanning two-dimensional scenes with alinear array camera are mostly dependent on either physically moving ortranslating the object to be scanned, physically moving or translatingthe entire camera and lens assembly; or optically redirecting the fieldof view of the camera and lens. by use of a single rotating scanningmirror; or an assembly of multiple mirrors in a rotating or translatingconfiguration.

[0004] In the first two cases, translation of either the object or thecamera and lens requires moving relatively large masses over relativelylong distances. Mechanical realization of these systems results inexpensive and slow mechanisms. For the third case, mirror rotationintroduces at least two forms of scan error. One form of scan error isnon-linearity due to converting rotational motion into linear motion.The second form of scan error is defocusing due to a change in opticalpath length either between the object and lens or the lens and the imageplane.

[0005] In general, for any prior art scanning mirror or prism system,one of the two scan errors described occurs and must usually becorrected in some manner such as with the use of additional movingcorrecting mirrors or with limited utility f-theta lenses.

[0006] Several techniques are currently available for optically scanningan object or scene. However, all known methods suffer either from scanerrors, or require movement of relatively large masses.

[0007] An object of the present invention is to produce a linear arraycamera having a linear optical scanning device that allows theoreticallyperfect linear translation of an image without defocus or other scanerrors.

[0008] Another object of the present invention is to produce a lineararray camera having a linear optical scanning device that may beconstructed having a smaller size than equivalent performance devices ofprior art, and allows theoretically perfect linear scanning of a largeobject field.

[0009] Another object of the present invention is to produce a lineararray camera having a linear optical scanning device whereby adaptationfor use with a large variety of lens types and focal lengths issimplified.

[0010] Still another object of the present invention is to produce alinear array camera having a linear optical scanning device wherebyinsertion of the linear optical scanning device into the image spacebetween a lens and a camera or detector, when used with lenses with longback focal lengths, is simplified.

SUMMARY OF THE INVENTION

[0011] The above, as well as other objects of the invention, may bereadily achieved by a linear optical scanner comprising: a mirrorassembly including a plurality of mirrors, each of the mirrors having areflective surface; and a drive means operatively connected to themirror assembly for mechanical translation of the mirror assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above, as well as other objects, features, and advantages ofthe present invention will be readily apparent to those skilled in theart from reading the detailed description of the preferred embodimentsof the present invention when considered in light of the accompanyingdrawings, in which:

[0013]FIG. 1 is a schematic view illustrating a linear array camerarequiring object or image motion relative to the linear arrayphoto-detector;

[0014]FIG. 2 is a schematic view of a linear optical scanner,illustrating two forms of scan error: non-linearity due to convertingrotational motion into linear motion, and defocusing due to a change inoptical path length between the object and lens or the lens and theimage plane;

[0015]FIG. 3 is a schematic view of a linear optical scannerincorporating the three-mirror configuration of the present invention;

[0016]FIG. 4 is a schematic view of a linear optical scannerincorporating the features of the present invention whereby the mirrorassembly is fixed at a distance dl from the optical axis;

[0017]FIG. 5 is a schematic view of a linear optical scannerincorporating the features of the present invention whereby the mirrorassembly is fixed at a second distance d2 from the optical axis;

[0018]FIG. 6 is a schematic view of a linear optical scannerincorporating the features of the present invention whereby the mirrorassembly is fixed at a third distance d3 from the optical axis; and

[0019]FIG. 7 is a plan view of a linear array camera incorporating thefeatures of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] Referring to the drawings, FIGS. 1 and 2 illustrate the prior artshowing the basic concept of requiring object or image motion relativeto the linear array photo-detector. There is also disclosed two types oferrors, specifically showing the effects with a single scanning mirrorrotating about an axis into the plane of the page. The mirror is shownat three equally spaced angular positions with 1 being equal to 2. Foreach angular position, the point of best focus in object space is shownas p0, p1 and p2. The vertical distances between each of these pointsare not equal and in general change non-linearly as a function of themirror angle . Likewise, the horizontal distances df1 and df2 from theobject plane are shown to be non-zero, increasing non-linearly asincreases.

[0021] The present invention utilizes a simple three-mirror assembly ina configuration sometimes known as an Abbe k-mirror. FIG. 3 disclosessuch an assembly which is placed between a lens L, and the image planeIP of the lens. The three mirrors are arranged as follows: thereflecting plane of the first mirror M1 is disposed to make an anglewith the optical axis OA of the lens. The reflecting plane of the secondmirror M2 is parallel to the optical axis of the lens, and at a distances from the upper edge of M1, as shown. The reflecting plane of the thirdmirror M3 is also disposed to make an angle with the optical axis,wherein the angle is equal in magnitude, but opposite in sign to theangle formed between the mirror M1 and the optical axis OA. Usually theupper edge of the mirror M3 is coincident with the upper edge of themirror M1. Such a k-mirror assembly has previously been used to providea means of rotating images in a manner similar to that of a Dove Prism.This is achieved by rotating the assembly about the optical axis OA. Forthe present invention, the k-mirror is translated only, and in adirection that is parallel or anti-parallel to the normal N of thesingle plane mirror M2.

[0022] Also shown in FIG. 3 are: object points O1, O2, and O3 that lieon the object plane OP; and conjugate image points I1, I2, and I3 thatlie on the image plane IP as it would exist if the mirror assembly werenot in place.

[0023] In general, when such a mirror assembly is placed between a lensand an image plane and translated by a distance d in a direction that isparallel or anti-parallel to N, the image is shifted in the samedirection, but by a distance that is exactly equal to twice d. FIGS. 4,5, and 6 illustrate an example of the case for three different positionsof the mirror assembly with the proper reflections from the mirrorsbeing shown.

[0024]FIG. 4 shows the position of the mirror assembly where thedistance in the N-direction from the mirror M2 to the optical axis OA isdl. In this case, the reflected path of the image space light rays isshown such that I1 still lies on the optical axis OA, but is translateda distance D from the original image plane IP onto a new image planeIP′. The new position is noted as I1′ and is simply the result of theadditional mechanical path length introduced by the k-mirror assembly.The image points I2 and I3 are likewise shown to be translated to thenew image plane IP′ and have been designated as I2′ and I3′. Inaddition, the relative positions of all three new image points have beeninverted.

[0025] It must be noted that in actual fact for the specific arrangementshown, the light rays contributing to I3′ would not actually converge tothe point shown. The I3′ rays reflect from the mirrors M1 and M2, andthen strike the mirror M1 again instead of the mirror M3, then themirror M2 again, eventually converging to the real image point P3′. Thisis because the k-mirror assembly as represented is not large enough. Therays I3′ are thus lost to the image plane IP′. Nevertheless, the graylines are shown where these rays would have converged had they struckthe mirror M3 immediately after being reflected from the mirror M2.

[0026] Also shown in FIG. 4 is the location of a possible linear arrayLA of photodetectors that is chosen to be coincident with I1′. Thelinear array LA is disposed such that the long axis of such aone-dimensional detector device is shown extending into the page. Thetype of detector has no receiving elements above or below the opticalaxis as shown. Thus, the loss of the rays I3′ to the image plane IP′ isirrelevant, as these rays do not contribute to detectable light for theillustrated position of the k-mirror.

[0027]FIG. 5 shows the mirror assembly having been moved anti-parallelto N so that the mirror M2 now lies a distance d2 from the optical axisOA. The distance moved, d1-d2, is chosen to be exactly half the distancebetween I1′ and I2′. The reflected path of the image space light raysnow shows that all three image points I1′, I2′, and I3′ have beenshifted anti-parallel to N by a distance equal to twice d1-d2, and theimage point I2′ is exactly coincident with the linear array LA.

[0028] Finally, FIG. 6 shows the mirror assembly having been movedanti-parallel to N so that the mirror M2 now lies a distance d3 from theoptical axis OA. The distance moved d1-d3 is chosen to be exactly halfthe distance between the image points IP1′ and IP3′. The reflected pathof the image space light rays shows that all three image points havebeen shifted anti-parallel to N by a distance that is twice d1-d3, andthat the image point I3′ is exactly coincident with the linear array LA.

[0029] Again, it must be noted that in actual fact some of the imagepoint light rays are lost to the image plane IP′. As illustrated some ofthe rays contributing to I1′ do not reach IP′. The gray line shownindicates where one I1′ ray would have converged had it not missedstriking M1 initially instead of converging to point P1′ shown. Again,however, these rays would not contribute to light detection at LA, andso the loss is irrelevant.

[0030] The description and the drawings include light rays tracing fromthe upper half of a full set of object points, assuming that the opticalsystem is viewing the center of an object. Lack of inclusion of otherobject points is solely to maintain clarity and comprehensibility ofdisclosure. It has been found that movement of the k-mirror assembly ina direction parallel to N will result in exactly the same type of imageshift in the N-direction.

[0031] The proof of the concept of the present linear optical scanningdevice may be found by simple geometric ray tracing of light raysreflected from the three mirrors of the k-mirror assembly. Moderncomputer automated design (CAD) software programs may be used to modelthe system with almost any degree of accuracy desired. The limitation isonly dependent on the inherent accuracy of the software. Such modelingshows that given a perfect lens, perfect construction of the k-mirror,and perfect mechanical translation mechanism, the image shift is alwaysexactly equal to twice the k-mirror shift, and there is no defocuswhatsoever at the image plane.

[0032] The present invention obviates the need for correction becausethe scan errors do not occur. Furthermore, the scanning may be achievedwith a relatively small, low mass mechanism. Such a feature providessignificant advantage over the previously described method ofmechanically translating the object or the camera and lens.

[0033] Furthermore, other prior art devices that produce “flat field”linear scanning require either complex cam and rack mechanisms designedfor a specific application, such as, for example, U.S. Pat. No.5,058,968 to Stark, or are limited to object space telecentric imagingthat is not suited for large object scanning, U.S. Pat. No. 4,647,144,to Finkel. Also, in these cases, the corresponding mechanisms are notsuited for simple, broad use with a wide variety of lenses.

[0034] In the preferred embodiment of the present invention, athree-mirror k-mirror configuration as has been shown, with the anglesbetween M1 and the optical axis being 150 degrees, the angle between M3and the optical axis being −150 degrees, and the angle between M2 andthe optical axis being zero degrees. Other angles and otherconfigurations are possible, including those with an odd number ofmirrors greater than 3. The described simple k-configuration, however,appears to require the least amount of optical path length, thus makingit more suited for use with a broader range of lenses.

[0035] Further, the preferred embodiment of the present invention allowsfor use of the linear optical scanning device with lenses that haverelatively long back focal lengths. Because of the optical path lengththat even simple multi-mirror arrangements require, longer back focallengths are preferred.

[0036] The preferred embodiment of the present invention also utilizes alens that provides flat-field imaging and low optical distortion. Thelinear optical scanning device itself is capable of perfect linearperformance with no focus error. Therefore, a lens that does not producea flat field and low distortion (neon-linear imaging) in the first placedefeats some of the purposes of the linear optical scanning device. Twoexamples of good lenses for use with the linear optical scanning deviceare high quality photographic objectives, and photographic enlargerlenses.

[0037] Additionally, the preferred embodiment of the present inventionutilizes lenses operated at relatively high F-number/small numericaperture settings. As the working F-number of the lens decreases, thesolid angle of cones of light coming from the exit pupil of the lensincreases. As this occurs, the scanner mirror assembly must be madelarger in order to collect all the useable light and prevent vignetting.Enlarging the scanner mirror assembly will tend to negate the advantageof having a compact linear optical scanning device design. Furthermore,additional mechanical path length is required for larger mirrorassemblies.

[0038] The preferred embodiment of the present invention furtherutilizes a highly linear mechanical motion of the k-mirror assembly.Again, the image motion is exactly twice that of the mechanical motion.Any mechanical errors will be reflected in the image motion, and withtwice the magnitude. It is understood, however, that there may beapplications in which nonlinear image motion is desired.

[0039] In order to prevent stray or direct light from reaching the imageplane, light blocking baffles should be used in conjunction with thek-mirror assembly.

[0040] For broadest utility, the linear optical scanning device shouldbe housed in an enclosure that allows easy use with a variety of lenstypes and focal lengths, as well as a variety of camera types.

[0041] Referring now to FIG. 7, there is shown generally at 10 a lineararray camera illustrating a preferred embodiment of the presentinvention. The linear array camera 10 includes a linear optical scanner12 having a first mirror 14, a second mirror 16, and a third mirror 18disposed on a mounting plate 20. The mounting plate 20 is operativelydisposed on a plurality of slide rails 22. The plurality of slide rails22 are disposed on a frame 24 and slidingly disposed on the mountingplate 20. Additionally, a plurality of light baffles 26 are disposed onthe mounting plate 20. A drive screw 28 is centrally disposed in theframe 24 and rotatably connected to the mounting plate 20. A drive means30 is disposed on the drive screw 28. A camera adapter means 32 isdisposed adjacent to the frame 24. A camera lens adapter 34 is disposedadjacent to the frame 24 opposite the camera adapter means 32. A cameralens 36 is disposed within the camera lens adapter 34. A linear array ofone-dimensional photo-detectors 38 is disposed adjacent to the cameraadapter means 32 and opposite the frame 24. The linear array camera 10is further disposed within the hollow interior of a housing 40.

[0042] While specification has been made to a screw drive means, it willbe understood that the other drive means may be satisfactorily employed.The drive means may consist of a stepper motor, servomotor or any otherrotating motor coupled to a drive screw that moves the mirror assembly.A rotating motor may also drive a rack and pinion arrangement, or adirect contact cam whose shape is designed to convert rotary motion intolinear motion. One such possible cam shape is the so-called “Spiral ofArchimedes”. Direct linear drives may also be employed. Such drives mayinclude linear motors in which the otherwise conventional stator androtor are “unwrapped” into straight lines, direct linear piezoelectricactuators, voice coil actuators, or any other type of linear actuatorsuch as solenoids or even pneumatic or hydraulic cylinders.

[0043] From the foregoing description, one ordinarily skilled in the artcan easily ascertain the essential characteristics of this inventionand, without departing from the spirit and scope thereof, can makevarious changes and modifications to the invention to adapt it tovarious usages and conditions in accordance with the scope of theappended claims.

What is claimed is:
 1. A linear optical scanner comprising: a mirrorassembly having an optical axis said assembly including at least threemirrors arranged in an Abbe K-configuration; and drive means operativelyconnected to said mirror assembly for effecting mechanical translationof said mirror assembly along a path perpendicular to the optical axisof said mirror assembly.
 2. The invention defined in claim 1 whereineach of the mirrors of said mirror assembly has a reflection surface. 3.The invention defined in claim 2 further including a lens having anoptical axis, and a spaced apart image plane wherein said mirrorassembly is disposed parallel to the optical axis of said lens.
 4. Theinvention defined in claim 3 wherein light rays passing through saidlens will produce an image at an image plane.
 5. The invention definedin claim 4 including a linear detector is disposed at the image plane.The lens, mirror assembly and image plane do not “co-operate to form alinear detector.” The linear detector is a type of sensor that may beplaced at the image plane.
 6. The invention defined in claim 5 whereinthe linear axis of said linear detector is perpendicular to the opticalaxis of said lens.
 7. The invention defined in claim 6 wherein saiddrive means is effective to translate said mirror assembly in adirection perpendicular to the optical axis of said lens and the linearaxis of said linear detector.
 8. The invention defined in claim 7wherein the light rays passing through said lens and contributing toform an image at an image plane are reflected off of each of thereflective surfaces of the mirrors of said mirror assembly prior toconverging to form an image at the image plane.
 9. The invention definedin claim 5 wherein translation of said mirror assembly in a directionperpendicular to the optical axis of said mirror assembly and said lensand perpendicular to the linear axis of said linear detector will resultin translation of the image.
 10. The invention defined in claim 9wherein the velocity of translation of the images will equal twice thevelocity of translation of said mirror assembly.
 11. The inventiondefined in claim 10 wherein the focus of the light rays passing throughsaid lens remain constant during translation of said mirror assembly.